The following techniques for protecting visual information against unauthorized access and modification are known:
A. Digital cryptographic techniques in which the visual information is first digitized. The result can be digitally encrypted and digitally signed by known techniques. This technique is expensive, since the digital image requires a large memory and its processing requires a personal computer or similar device.
B. Exotic printing techniques in which visual information is printed with unusual inks on unusual types of paper or other sheet material. These techniques are often used to print money, stock certificates and official documents. Verification of whether a document is forged may require a well-stocked laboratory and well-trained personnel and is not cost effective for small batches of privately produced documents.
C. Holograms, embossed letters and erasure-resistant backgrounds are often used for credit cards or similar devices to deter forgeries. Criminals can easily overcome such protective mechanisms.
D. Code books may be used for encoding a small number of possible images by using secret numeric equivalents which are printed either together with or instead of the actual image. Such a scheme is practical only if the list of possible images is fixed in advanced and the coding technique is very simple. Such a system cannot be adapted to arbitrary images.